1. Field of the Invention
The present invention relates to an error correction device and an error correction method for correcting errors in data items sent on a plurality of channels.
According to the present invention, data items sent on a plurality of channels refer to data items to be handled in diverse fields such as: the field of the asymmetric digital subscriber line (ADSL) and symmetric digital subscriber line (SDSL) technologies that utilize wire communication as a transmission medium; the field of the orthogonal frequency division multiplexing (OFDM) technology that utilizes radiocommunication as a transmission medium; the field of the wavelength division multiplexing (WDM) technology that utilizes an optical fiber as a transmission medium and adopts light having a plurality of wavelengths; and the field of the technology for performing parallel recording or reproducing of data using a recording medium.
2. Description of the Related Art
Various types of data transmission systems are already known. For example, transmission systems utilizing a power line as a data transmission line are also known. FIG. 22 shows a power line carrier communication system that is one type of transmission system utilizing the power line. There are shown a distribution substation 101, an access node 102, a high-tension distribution line 103, a pole transformer 104, a low-tension distribution line 105, a drop wire 106, and house wiring 107.
A high ac voltage of, for example, 6.6 kV is distributed from the distribution substation 101 to each pole transformer 104 over the high-tension distribution line 103. The high voltage is stepped down to 100 V or 200 V and is supplied to households or any other users. This causes various kinds of electric equipment coupled to the house wiring 107 or various kinds of electric equipment plugged into outlets to start operating.
The access node 102 installed in the distribution substation 101 and a modem (not shown) incorporated in the pole transformer 104 are linked by an optical fiber transmission line (not shown). The optical fiber transmission line is generally laid along the high-tension distribution line 103. The modem in the pole transformer 104 transforms a light signal into an electric signal or vice versa. The low-tension distribution line 105, drop wire 106, and house wiring 107 are used as a data transmission line for wire communication. Once a terminal is plugged into an outlet coupled to the house wiring 107, a power line carrier communication system, called a “Last One Mile” system, can be constructed for communication of data between the access node 102 and the terminal.
In this kind of power line carrier communication system, the low-tension distribution line 105 offers an inductive impedance to the flow of an alternating current from the modem in the pole transformer 104, and the drop wire 106 and house wiring 107 offer a capacitive impedance thereto. Moreover, various kinds of electric equipment coupled to the house wiring 107 generally have an anti-noise capacitor incorporated therein. Therefore, the impedance to the flow of an alternating current from the modem in the pole transformer 104 includes a relatively large inductive reactance and a large capacitive reactance.
Consequently, for the modem in the pole transformer 104, the low-tension distribution line 105 is comparable to a low-pass filter. A signal received by a modem coupled to the house wiring 107 has the high-frequency components thereof reduced greatly. Namely, the high-frequency components of the received signal are buried under noise. Moreover, the low-frequency components of the received signal do not decay as much as the high-frequency components do. A random noise sent from a switching power supply or an inverter circuit incorporated in electric equipment is delivered to largely affect the received signal.
For example, referring to FIG. 23A, the axis of the ordinate indicates power levels PWR and the axis of the abscissa indicates frequencies. A dot-line curve expresses a noise-canceling characteristic, and solid-line curves indicate received signal levels and noise levels. By utilizing the noise-canceling characteristic expressed with the dot-line curve, the noise level of low-frequency components can be made lower than the received signal level. However, as far as electric equipment employing an inverter is concerned, noise having a comb-like wave are often distributed over a broad frequency band. In this case, for example, as shown in FIG. 23B, high noise levels are detected outside a noise-canceling band. This leads to the frequent occurrence of errors in the received data.
The orthogonal frequency division multiplexing (OFDM) technique is one of the techniques for transmitting data using multiple carriers. The carriers are orthogonal to one another. Multiplexing is performed using the multiple carriers. It is therefore possible to assign frequencies, which do not fall within frequency bands causing high noise levels, to the carriers. The discrete multitone (DMT) technique is one of the techniques for transmitting data using a plurality of carriers, and is adopted as a modulating technique to be adapted to the asymmetric digital subscriber line (ADSL) technique.
FIG. 24 is an explanatory diagram concerning a data transmission device proposed previously. The data transmission device is equivalent to the modem that is included in the aforesaid power line carrier communication system and connected to the house wiring in order to transmit or receive data. Referring to FIG. 24, there is shown a code converter 111 having a scrambling (SCR) capability, a series-to-parallel (S/P) conversion capability, a Gray code-to-natural binary (G/N) conversion capability, and a finite sum arithmetic capability.
There are also shown a signal element generation unit 112, an inverse fast Fourier transform (IFFT) unit 113 having a guard time GT addition capability, a zero element insertion unit 114, a rolloff filter (ROF) 115, a modulator (MOD) 116, a digital-to-analog (D/A) converter 117, a low-pass filter (LPF) 118, a transmission clock generation unit (TX-CLK) 119, a transmission line (TX-line), and a reception line (RX-line). Moreover, there are shown a bandpass filter (BPF) 120, an analog-to-digital (A/D) converter 121, a demodulator (DEM) 122, a rolloff filter (ROF) 123, a reception clock distribution unit (RX-CLK), a timing sample unit (TIM) 125. Furthermore, there are shown a phase locked loop (PLL) 126 including a voltage-controlled crystal oscillator (VCXO), a noise removing unit 127, a fast Fourier transform (FFT) unit 128 having a guard time (GT) deletion capability, a signal identification unit (DEC) 129, and a code converter 130 having a difference arithmetic capability, a natural binary-to-Gray code (N/G) conversion capability, a parallel-to-series (P/S) conversion capability, and a de-scrambling (DSCR) capability. Moreover, a transmission signal SD and a reception signal RD are also shown.
A clock pulse generated by the transmission clock generation unit 119 is applied to the circuit elements. To the zero element insertion unit 114, the clock pulse is applied as a timing signal that determines the timing of inserting a zero element. The code converter 111 scrambles the transmission signal SD, and converts it into the same number of frequency components as the number of carriers so that the frequency components will be transmitted in parallel with one another. Moreover, the code converter 111 converts the number system adopted for the transmission signal SD converted from the Gray code into the natural binary. Moreover, the code converter 111 performs finite sum arithmetic so that a receiving side can perform difference arithmetic. Thereafter, the signal element generation unit 112 gives a Nyquist interval to the signal elements. The inverse FFT unit 113 adds a guard time GT to the resultant signal and performs inverse FFT on the signal. The zero element insertion unit 114 inserts zero elements having a zero level according to the timing signal that determines the timing of inserting zero elements. The rolloff filter 115 re-shapes the waveform of the signal. The modulator 116 digitally modulates the signal. The D/A converter 117 converts the signal into an analog form. The low-pass filter 118 places low-frequency components, which fall within a transmission band ranging, for example, 10 kHz to 450 kHz, on the transmission line TX-line. In this case, the transmission line TX-line and reception line RX-line are linked with the house wiring and coupling filters between them.
The reception clock distribution unit 124 distributes a clock pulse, which is produced based on a clock pulse sent from the phase locked loop 126, to the circuit elements. A signal received over the reception line RX-line has the frequency components thereof, which fall within, for example, a range from 10 kHz to 450 kHz, passed through the bandpass filter 120. The A/D converter 121 digitizes the signal. The demodulator 122 demodulates the signal. The rolloff filter 123 re-shapes the waveform of the signal. The noise removing unit 127 detects the noise levels of noises superposed on zero elements according to the clock pulse sent from the reception clock distribution unit 124, interpolates the noise levels to calculate the noise level of a noise superposed on a signal element, and removes the noise from the signal element. The FFT unit 128 deletes the guard time GT, and transforms the signal from the time domain into the frequency domain. The signal element identification unit 129 identifies the signal. The code converter 130 performs parallel-to-series conversion, de-scrambling, difference arithmetic, and natural binary-to-Gray code conversion so as to produce the reception signal RD.
FIGS. 25A to 25D are explanatory diagrams concerning a zero element inserted by the zero element insertion unit 114 shown in FIG. 24, and noise removal. Referring to FIGS. 25A to 25D, the transmission speed for signal elements S (25A) shall be 192 kB, and zero elements indicated with black dots are inserted into a transmission signal (25B). By copying one bit, one zero element can be equivalently inserted between signal elements S. The insertion of zero elements doubles the transmission speed to 384 kB. A received signal (25C) has a noise N superposed on the signal elements S and zero elements respectively during transmission. The noise N superposed on the zero elements is sampled. As the same noise as the sampled noise N is superposed on the signal elements, the noise N is removed from the signal elements. Consequently, a reception signal or a signal resulting from noise removal (25D) can be restored.
Incidentally, the insertion of zero elements is such that one zero element may be inserted among a plurality of signal elements or a plurality of zero elements may be inserted between signal elements. For example, when two zero elements are inserted between signal elements, if raw data falls within a frequency band of 128 kHz wide, the frequency band is expanded to be 384 kHz wide.
Generally, an error correcting means for data appends an error correcting code to data, and detects, based on the error correcting code, if an error has occurred in the data. If an error has occurred, the error is corrected. However, since the error correcting code is composed of a plurality of bits, the appending of the error correcting code may lead to the deterioration of the efficiency in high-speed transmission. This poses a problem.
Moreover, as mentioned above, when the means for inserting zero components, sampling a noise superposed on the zero components, and canceling a noise superposed on signal components according to the sampled noise is adapted in the power line carrier communication system, high-speed transmission can be achieved with the adverse effect of noises minimized. However, the distribution of noise is, as shown in FIG. 23B, observed over a plurality of frequency bands, and the noise levels thereof are relatively high. Moreover, the noise levels and frequency bands often vary time-sequentially. Consequently, the noise components cannot be removed reliably. This leads to occurrence of errors in identifying data.
When multilevel modulation is adopted, the modulated signal elements of a received signal vary greatly due to the adverse effect of noises. An error in identifying data occurs frequently. This poses a problem in that it is hard to increase the number of signal levels to be modulated through multilevel modulation that enables high-speed transmission.